Desulfurization of petroleum hydrocarbons with aqueous caustic soda and formaldehyde



C. W. RIPPIE OF PETROLEUM HYDROCARBONS WITH AQUEOUS CAUSTICSODA ANDFORMALDEHYDE Filed March 18, 1958 Sept. 13, 1960 DESULFURIZATION am NNSDWMQ United States Patent yOffice 2,952,627 Patented Sept. 13, 1060DESULFURIZATION OF .PETROLEUM HYDRO- CARBONS WITH AQUEOUS CAUSTIC SODAAND FORMALDEHYDE Charles W. Rippie, Ashland, Ky.,

Oil & Refining Company, of Kentucky Filed Mar. 18, 1958, Ser. No.722,212

6 Claims. (Cl. 208-231) This invention relates to the purication ofvpetroleum hydrocarbons and is directed particularly to thedesulfurization thereof.

A principal objective of this invention has been to provide a process bymeans of which sulfur compounds present in liquid hydrocarbons such asgasoline may be removed in an easy and inexpensive way.

A further objective of the invention has been to provide a process forlimproving the lead susceptibility and stability of hydrocarbon motorfuels such that a given octane number may be -achieved by theincorporation of a relatively smaller amount of tetraethyl lead and suchthat the amount of inhibitors conventionally added may be reduced.

The adverse affect of small quantities of sulfur compounds contained ina hydrocarbon motor fuel upon the lead suscept-ibility 4thereof isWell-understood, and various processes are now used to reduce suchsulfur content to a minimum consistent with the cost of treatment andthe savings in lead cost to be obtained thereby. However, it isgenerally impractical or impossible for the reflner to endeavor toobtain complete desulfurization because the processing cost would exceedthe savings that would be derived from the improvement in leadsusceptibility. While the sulfur content of liquid hydrocarbon motorfuels varies according to the method by which they are obtained, it isnot unusual for catalytically cracked gasoline or motor fuel fractionsto contain from 0.02 to 0.40% sulfur depending upon feed stock,catalyst, severity of processing, and other -conditions and forstraight-run gasolines to contain still more.

It has long been known that treatment of a liquid petroleum hydrocarboncontaining organic sulfur with caustic soda in aqueous solution iscapable of converting some of the sulfur compounds into substances whichare more soluble in the caustic solution than in the hydrocarbon andwhich therefore are removed from the hydrocarbon upon separation of thevaqueous alkali therefrom. The reaction and dissolution are rapid, andthe cost of treatment is reasonable inasmuch as the caustic sodasolution, after use, may be regenerated yfor reuse. However, causticsoda treatment is effective in removing only those sulfur compoundswhich are of the RSH type, e.g., mercaptans. Organic suliides of the R28type remain substantially unaffected, yet they constitute `a substantialpercentage of the total sulfur content and impair the leadsusceptibility of the fuel in which they are present.

An objective of the present invention has been to provide adesulfurizing process of the caustic soda type but modified to effectremoval of organic sulides of the R28 type simultaneously with removalof sulfur compounds of the RSH type contained in the pertoleumhydrocarbon. More specifically, an objective of the invention has beento provide a desulfurizing process of the caustic soda type wherein thetotal sulfur content may be reduced to a greater degree than with theuse of caustic soda alone.

Briefly, this invention is directed to a process of desulfurizationwherein liquid petroleum hydrocarbon Yconassignor to Ashland Ashland,Ky., a corporation taining sulfur compounds, but which is free orsubstantially free of hydrogen sulfide, is contacted conjointly with astrong aqueous solution of caustic soda and a small `amount offormaldehyde, whereby the total sulfur is reduced materially and thelead susceptibility of the product .is substantially improved incomparison with the lead susceptibility of the same hydrocarbon feedstock treated with caustic soda only.

' Although the use of formaldehyde previously has been .proposed as adesulfurizing agent in processes wherein strong acid, high temperature,or molten sodium has been employed, it Ihas now been found that asubstantial reduction of total sulfur readily can be achieved in a muchsimpler Way and Vat low or ordinary processing temperature by contactingthe organic sulfur-beating hydrocarbon simultaneously with caustic sodaand a small quantityrof formaldehyde, for instance l part formaldehydeto 15,000 .to 40,000 parts by volume of hydrocarbon being treated. Infact, an important feature of the present invention resides in the useof an amount of formaldehyde which, even Ywhen in commercial aqueoussolution is so small that it does not dilute the caustic soda solution`by a significant amount or adversely affect the regenerability thereof.

Peculiarly, treatment of hydrocarbon feed stock with caustic soda andformaldehyde simultaneously or conjointly provides a greater reductionof total sulfur content than that obtained by contacting the feed stocksuccessively with caustic soda only, then formaldehyde without causticsoda, `or vice versa. The chemical mechanism by which such results areobtained is not fully understood; it is possibly that the caustic soda,While converting mercaptans into mercaptides in the usual manner,promotes reaction of the formaldehyde with compounds of the R28 typewith which formaldehyde alone does not possess the `ability to react. Inlany event, the addition of an -apparently negligible quantity offormaldehyde in the presence of caustic soda has been found consistentlyto decrease the total sulfur content by an amount which is 20u-40%greater than is furnished by caustic soda alone, and permits 0.5 to 0.6cc. per gallon less tetraethyl lead to be added to the motor fuel thanis otherwiserequired to produce a given octane number in the `same stockdesulfurized by caustic soda only.

Formaldehyde and caustic soda in aqueous admixture react with one`another at ordinary temperatures to produce nsodium formate andalcohol, neither of which possesses desulfurizing properties. Therefore,in the practice Yof the present invention it lis desirable to introducethe formaldehyde linto contact With the `caustic: soda and hydrocarbonin such manner that desulfurization of the hydrocarbon -by the conjoint.action of the caustic soda and formaldehyde is accomplished before Itheformaldehyde is destroyed ythrough reaction with the caustic soda. Thismay be accomplished conveniently by introducing the formaldehyde :intothe caustic soda solution at a point just ahead of that :at ywhichcontact with the hydrocarbon is established, or by introducing theformaldehyde -at the lsite wherein the caustic soda is brought intocontact 'with the hydrocarbon. In either manner the yformaldehyde iscaused to react with organic sulfur components before undesirablepre-desulfurizing reaction occurs :between the formaldehyde and thecaustic soda. In general, however, the process of this invention, exceptas yto `addition of formaldehyde, may be the `same as a conventionalcaustic soda desulfurization process as practiced in ,the various waysknown :to the The invention is further predicated upon the discovery anddetermination that a spent caustic soda `solution in which formaldehydehas Ibeen incorporated to promote greater desulfurization of hydrocarbonin the treatment introduced into the system,

is more easily regenerated than when no formaldehyde has beenincorporated. For example, in a conventional caustic soda regenerationprocess wherein the suliide content of the spent solution is convertedIinto removable disuldes by blowing with air,.the spent solution must beheld at appreciable .temperature and must be blown for an appreciableperiod of time in order to provide a suitable degree of regeneration. Incontrast with these requirements, the sultdes contained in spent caustics-oda as produced in the desulfurization process of the presentinvention are converted into removable sulfur compounds 4in a rapidandeflicient manner Without increase in temperature, for` example,merely by incorporating air into the -spent caustic solutionY throughuse of an eductor or the like. The spent caustic solution produced in Vadesulfurizing process conducted according to the present inventionreadily may be regenerated by other known regeneration procedures.

One typical process of desulfurization followed by vregeneration `of thespent caustic solution is illustrated in the accompanymg drawing.Sulfur-containing gasoi Yline (or other hydrocarbon to be desulfurized)but from 4 uct, passes through line 8 to storage.V Desulfurization ofthe gasoline occurs in the period of transit of the stream to theknock-out drum and during'the time of residence therein while separationof the caustic soda solution from the gasolineoccrs. Spent vaqueouscaustic soda is removed from the bottom .layer in the knockout drumthrough line 9 which connects to receiver 10.

For regeneration of the spent caustic solution, a pump 12 having itsintake line connected to tank 10, discharges through line 13 to a tank14. However, to facilitate introduction of air into the stream, the pump12 operates in conjunction with a recirculation Ysystem 15 to which airis introduced in quantity suicient to cause conversion of organicsuliides in the spent caustic soda solution into disulfdes which may beremoved by washing as later described. As shown, Kthe recirculationsystem comprises a line 16 connected to the pump discharge line 13 and.which hydrogen sulfide has been removed by any suitable means entersthe system through =line 1 wherein it mixes with caustic soda solutionfrom the line 2. Aqueous Vformaldehyde Ysolution is admitted to thestream .through line 3. The caustic soda solution provided in line 2 maybe fresh aqueous caustic soda solution or may be a solu- 'tion which hasbeen regenerated in various ways, one of which subsequently described.

' In typical caustic sodav desulfurization, the aqueous caustic.solution used may contain approximately 10% by weight of caustic soda(or caustic potash) in water, and the ow rate is adjusted to provideapproximately one partby` volume yof caustic soda solution to five partsby volume of gasoline, but it is runderstood that substantial variationsin the causticgasoline ratio may be made to suit the requirements of`given feed stocks. In the preferred practice of the invention, theformaldehyde employed is a commercial 37% aqueous formaldehyde solutionknown in the trade as formalin. .This solution may be metered into thecaustic soda stream solution of ,line 2 at a rate of one part by volumeof formalin to about 15,000-18,000 parts by volume of sour `gasolinecharged into the system through line 1. However, in place of usingIformaldehyde as a 37% aqueous solution, which was chosen because of itscommercial availability and cheapness, other concentrations may be used.VIn general, no economic benefit has been found in using formaldehyde ingreater proportion to the gasoline or caustic soda than that indicated.In the system shown, the formaldehyde flow rate :is controlled by meansof arproportioner 4 or suitable pump andY metering valve. Assubsequently explained, the pressure in iline 2 may be approximately200' p.s.i.

The stream containing sour gasoline, caustic soda solution, andformaldehyde' flowing in line 5 passes through a diiferential pressurecontrol valve indicated generally at 6, beyond which the stream passes-into a knock-out or settling drum 7. A-t the pressure control valve 6the pressure on the stream is reduced to approximately psi., and thevalve functions as a mix'uing valve bringing the components of thestream into intimate contact with one another. It will be seen that thepoint at which formaldehyde is introduced into the system Ais relativelycloselyY `adjacent the point lat which sour lgasoline is whereby theformaldehyde and caustic soda contact the Vgasoline before any signicantreaction occurs Vbetween thecaustic soda and the formaldehyde. Ifdesired, however, the lformaldehyde may be introduced `directly intoline 11 or all three components may be commiugled at a single point eig.the mixing valve 6. Y

At the knock-out dru-m 7, aqueous caustic soda settles from the gasolinewhich, as a now desulfurized proda line 17 which is connected to theline 16 and which leads to the intake side of pump 12. Line 16 is valvedas at 18 to cause a predetermined portion of the pump eiuent torecirculate through the line 17, and air may be introduced into therecirculating stream by means of a conventional eductor 19 before thestream is returned to the system. In place of the eduetor, air may beintroduced into the caustic soda stream in any other suitable manner.'Ihe significant point is that conversion of suliides to disulfides inthe spent caustic soda solution Vis very rapid. Some of the caustic sodasolution from pump 12 bypasses the eductor system, but the air absorbedinto the recirculated stream is adequate to effect conversion todisulfides of the entire spent caustic soda solution through thecommingling which occurs in the pipes, pump and tank 14.

A pump 20 delivers the air-treated caustic soda solution from tank 14through a line 21 to a disulfide scrubber 22 wherein the solution iswashed with a solvent, such as the conventionally used naphtha, capableof dissolving the disuldes therefrom. Y

In the scrubber system shown, a column of naphtha is maintained withintank 22 above a lower layer of caustic soda solution. To facilitatewashing, the naphtha phase is recirculated into contact with the causticsoda stream owing through pipe 21; for this purpose the re- Ycirculatingsystem includes an outlet line 23 which is located above thecaustic-naphtha interface and which leads to a pump 24. The pumpdischarges'through a line 2S,

into line 21, and both streams pass through a mixer 26, Y

The naphtha phase containing disuliides may also be treated for reuse.In the system disclosed, naphtha is withdrawn from the upper part of thedisulfide scrubber 22 through a line 30 which leads to a knock-out drum31, wherein entrained caustic soda solution is allowed to settle fromthe naphtha as a lower layer which may be withdrawn as necessary." Thesolvent is discharged from the knock-out drum through aline 32 leadingto a ash tower 33 which may be suitably packed. VTo facilitatevaporization of naphtha, the lower portion of the ash tower 33 may bemaintained at a temperature of approximately 275 F. by recirculation ofnaphtha through a steam reboiler34 which is supplied from line 35 andwhich returns heated Yliquid to thentiash tower through line 36. Byvaporization, the upper portion of the flash tower is kmaintained at asuitably lower temperature, for example,

200 F. whereby desulfurized naphtha vapors are generated which leave thetower through line 37 and pass to a condenser 38.

Condensed naphtha is collected in an accumulator tank 39 from which aline 40 returns the desulfurized naphtha solution rto the disulfidescrubber. In the system shown, the accumulator tank is provided with aliquid level controller indicated generally at 41 which controls themotor of a pump 42 installed in line 40 whereby the pump deliversnaphtha from the accumulator to the disulfide scrubber in accordancewith the rate at which it collects in the accumulator. Fresh naphtha maybe charged into the system through the line 43.

Itis also to be noted that the system for feeding naphtha to thereboiler 34 includes an outlet 44 which is valved as at 45. Sincedisulides collect at the bottom of the flash tower, the higher heat ofvaporization is employed as a means of maintaining the desiredtemperature diiferential in the ash tower. By the manipulation of valve45, disuliides at the bottom of the ash tower readily may be dischargedfrom the system.

In the practice of the process of this invention desulfurization of thehydrocarbon proceeds rapidly at or about room temperature, for examplefrom 70 to 100 F. While the rate of oxidation of sulfur compoundscontained in the spent caustic may be increased by increasing thesolution temperature during regeneration, that procedure is not requiredwhen the spent caustic solution contains spent formaldehyde since theoxidation rate is sufficiently rapid for practical purposes withouttemperature increase. Also, while the system has been shown for use indesulfurizing straight-run or cracked gasolines,

f it is to be understood that the latter term is intended to includemotor fuels produced by alkylation, reforming, isomerization, or othermethods. It is also to be understood that the desulfurizing advantagesafforded by the use of formaldehyde with caustic soda may be utilized innonregenerative methods of caustic treatment where the organic sulfurcontent is suciently low to permit wasting of the spent treatingsolution. Similarly, the invention is adapted to be used for removingsulfur from hydrocarbons heavier than fractions burning in the gasolinerange, such as fuel oils, in which organic suliides seriously interferewith utilization.

In typical operation of the process for desulfurizin-g gasoline, theaverage research octane number of the treated i.e. desulfurized gasolinewith 3 cc. of tetraethyl lead added per gallon and `adjusted to 8# RVP,was 84.35 when caustic soda only was used for desulfurization, while anoctane number of 85.80 (based on the same TEL addition) was obtainedwhen caustic soda and formaldehyde were used for desulfurization of thesame hydrocarbonan increase of 1.45 octane. Thus, the conjoint use offormaldehyde and caustic permits a saving of 0.6-0.7 cc. or more oftetraethyl |lead per gallon of gasoline. In terms of cost at the currentprice of tetraethyl lead, the amount thereof required to produce thegiven octane number was $56.55 per thousand barrels, while the cost offormaldehyde required for the treatment of 1000 barrels was $2.75. Thecost of regeneration is the same in either case.

The following table shows the comparison of total sulfur content ofgasolnes desulfurized with caustic soda and with caustic soda andformaldehyde in accordance with the present invention:

6 Extensive tests have shown further that on an average treatment withcaustic soda and formaldehyde reduces RSH sulfur by 35.5%, RZS surfur by23.8%, and total sulfur by 23.4%, and less inhibitor, therefore, isrequired to meet specifications for gums of the treated gasoline.

Tests Vof gasoline desulfurized according to the invention also showthat stability, as measured on copper and glass dishes, is improved, andless inhibitor therefore is required to meet specifications for gums ofthe treated gasoline.

In place of using naptha to wash disuliides from the caustic sodasolution, other suitable solvents may be used as readily will beunderstood by those skilled in the art. Similarly, as previouslyexplained, it is to be understood that the recirculation system hereindisclosed is merely typical and that other systems for recirculatingspent caustic solution readily may be used. Thus, the introduction offormaldehyde into the caustic soda treating solution not only increasesthe degree of desulfurization over that which could be obtained by theuse of caustic soda ondy, but renders the spent caustic soda solutionmore amenable to regeneration.

Having described my invention, I claim:

1. The process of desulfurizing liquid petroleum hydrocarbon containingalkyl sulde type sulfur compounds which method comprises, contacting thepetroleum hydrocarbon with aqueous caustic soda solution and introducinginto contact with both components, while excludingoxygen, a quantity offormaldehyde in the proportion of 1 part thereof to each l5,000-l8,000parts of said hydrocarbon thereby to effect conjoint reaction of thecaustic soda and formaldehyde with alkyl sulfide type sulfur compoundspresent in the hydrocarbon, then separating from the petroleumhydrocarbon the said aqueous caustic soda solution and sulfur compoundsdissolved therein derived from said alkyl suliides, and regenerating theseparated caustic soda solution for reuse by contacting the same withair.

2. A method of treating a petroleum hydrocarbon to reduce the amount ofalkyl sulfide type sulfur compound impurities therein which methodcomprises, bringing streams of aqueous caustic soda solution and thesaid petroleum hydrocarbon into liquid contact with one another and,which excluding oxygen, introducing formaldehyde into the stream ofIaqueous caustic soda solution at fa point just ahead of the point ofliquid contact thereof with the hydrocarbon stream in the amount ofapproximately one part of formaldehyde to 15,000-18,000 parts by volumeof the said hydrocarbon whereby the formaldehyde reacts with sulfurcompounds in the said hydrocarbon before substantial reaction thereofwith the caustic soda occurs.

3. A method of treating a petroleum hydrocarbon containing RZS and RSHtype sulfur compounds to reduce the amounts thereof which methodcomprises, continuously, while excluding oxygen, bringing streams ofaquecus caustic soda solution and the said petroleum hydrocarbon intoliquid contact with one another and with a small quantity offormaldehyde in the amount of vapproximately one part of formaldehyde to15,00040,000 parts by volume of petroleum hydrocarbon whereby theformaldehyde and caustic soda yact conjointly to remove RgS and RSH typesulfur compounds from the hydrocarbon.

4. The method of claim 3 wherein the treatment is conducted atapproximately room temperature.

5. The method of claim 3 wherein the treatment is conducted atapproximately room temperature and wherein spent caustic soda solutionproduced in the treat'- nient is subsequently regenerated for reuse byintroduction of air therein.

6. The method of removing objectionable alkyl sulfide compounds fromhydrocarbon motor fuel which method in whichY reagentthe formaldehydeconstitutes approxmately 1 pan by weight per 15,000 to 40,000 parts of 5UNITED STATES PATENTS hydrocarbon being treated,V then regenerating the'caustic 2,616,832 Browder et ral Nov. 4, 1952 comprises, treating saidmotor fuel, While excludingV soda solution for reuseV by introducing airtherein, then" oxygen, with adesulfurizing reagent consisting of aqueousseparating the reagent from the hydrocarbon motor 11501., caustic sodasolution and aqueous formaldehyde solution References Cited in the me ofthigpatet

1. THE PROCESS OF DESULFURIZING LIQUID PETROLEUM HYDROCARBON CONTAININGALKYL SULFIDE TYPE SULFUR COMPOUNDS WHICH METHOD COMPRISES, CONTACTINGTHE PETROLEUM HYDROCARBON WITH AQUEOUS CAUSTIC SODA SOLUTION ANDINTRODUCING INTO CONTACT WITH BOTH COMPONENTS, WHILE EXCLUDING OXYGEN, AQUANTITY OF FORMALDEHYDE IN THE PROPORTION OF 1 PART THEREOF TO EACH15,000-18,000 PARTS OF SAID HYDROCARBON THEREOF TO EFFECT CONJOINTREACTION OF THE CAUSTIC SODA AND FORMALDEHYDE WITH ALKYL SULFIDE THPESULFUR COMPOUNDS PRESENT IN THE HYDROCARBON, THEN SEPARATING FROM THEPETROLEUM HYDROCARBON THE SAID AQUEOUS CAUSTIC SODA SOLUTION AND SULFURCOMPOUNDS DISSOLVED THEREIN DERIVED FROM SAID ALKYL SULFIDES, ANDREGENERATING THE SEPARATED CAUSTIC SODA SOLUTION FOR REUSE BY CONTACTINGTHE SAME WITH AIR.